10.ISCA-RJCS-2013-079

Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X
Vol. 3(6), 69-73, June (2013) Res. J. Chem. Sci.
International Science Congress Association 69
Kinetic Modeling of Anaerobic Co-digestion of Water Hyacinth and
Poultry Litter
Patil J.H.*
, Molayan Lourdu Antony Raj, Bhagyashree, Vantimar Dinesh and Rohith
Department of Chemical Engineering, R.V. College of Engineering, Bangalore-560059, INDIA
Available online at: www.isca.in
Received 18th
May 2013, revised 30th
May 2013, accepted 14th
June 2013
Abstract
The over-utilization of global energy sources is a major problem to the present and future world community. It has been
estimated that the fossil fuels would be exhausted in the next few decades. In today’s energy demanding lifestyle, there is
always a need for exploring and exploiting new sources of energy which is renewable as well as eco-friendly. Anaerobic co-
digestion is a technology that utilizes more than one organic waste to produce methane, which holds promise for the future
while simultaneously addressing ecological and agrochemical issues. In the present study anaerobic co-digestion of water
hyacinth and poultry litter has been carried out in 300 ml batch digesters with 8% total solids content for different retention
period. After completion of the retention period of individual digester, total solids, volatile solids, pH and biogas evolved is
been tabulated Variation of Biogas production, total solids, volatile solids, and pH has been studied as a function of time.
Anaerobic co-digestion of Water Hyacinth and Poultry Litter produced biogas yield of 0.3810 l/g volatile solids. A kinetic
model was proposed using integral method of analysis which revealed the anaerobic co-digestion of water hyacinth and
poultry litter follows a first order reaction with rate constant 0.026 day-1
.
Keywords: Anaerobic co-digestion, water hyacinth, poultry litter, volatile solids, kinetic model.
Introduction
Energy is one of the most important factors to global prosperity.
The dependence on fossil fuels as a primary energy source has
led to global climate change, environmental degradation, and
human health problems. It has been predicted that by the year
2040, the world will have a population of 9 to 10 billion people
that must be provided with energy and materials1
. Moreover, the
recent rise in oil and natural gas prices may drive the current
economy towards alternative energy sources. Anaerobic
digestion is a biological process that converts organic matter
into biogas which mainly consists of methane (55 to 75%) and
carbon dioxide (25 to 45%) with calorific value of 20 MJ/m32-3
.
The potential of this process is wide, because anaerobic
digestion can be applied to a large variety of biodegradable
organic waste and effluents from urban, industrial, or
agricultural origins.
Co-digestion is the simultaneous digestion of more than one
type of waste in the same unit4
. Advantages include better
digestibility, enhanced biogas production/methane yield arising
from availability of additional nutrients, as well as a more
efficient utilization of equipment and cost sharing4-6
. Studies
have shown that Co-digestion of several substrates, such as,
banana and plantain peels, spent grains and rice husk, pig waste
and cassava peels, sewage and brewery sludge, among many
others, have resulted in improved methane yield by as much as
60% compared to that obtained from single substrates7-10
.
Anaerobic co-digestion of water hyacinth and primary sludge
was explored and found to improve biogas yield significantly11
.
Biomethanation of water hyacinth treated with poultry litter
enhanced biogas yield12
. In the present study anaerobic co-
digestion of water hyacinth and poultry litter was carried out in
300 ml batch digesters with 8% total solids content for different
retention period and a kinetic model was proposed to explain the
kinetics of anaerobic co-digestion of water hyacinth and poultry
litter.
Material and Methods
Collection of Substrates: Water hyacinth was obtained from a
lake near Kengeri Upanagara (Bangalore, Karnataka,
India).Fresh poultry litter sample was obtained from Chandru
Poultry Farm, Ullal Upanagara (Bangalore, Karnataka, India).
Materials and Apparatus: The following materials and
apparatus were used for the purpose of this research: weighing
balance(Systronics), pH meter (Systronics), a mercury in glass
thermometer (range 00
C to 1000
C), muffle furnace, oven, mixer
grinder, temperature controlled water bath, water troughs,
graduated transparent glass gas collectors, tap water, rubber
cork, connecting tubes and Tap water.
Experimental set up: Experimental set up consists of a
constant temperature bath with a provision to maintain desired
temperature. A battery of digesters each of volume 300ml was
kept in the temperature bath which was maintained in the
mesophilic temperature range from 30°C to 35°
C. Each bio-
digester was connected to a graduated gas collector by means of
a connecting tube. Each of the gas collectors were in turn

Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606X
immersed in a trough containing water to ensure complete water
sealing. A stand held all the gas collectors such that the water
displacing hole at the bottom end immerses in water. Biogas
evolved was collected by the downward displacement of water.
The experimental set up for biomethanation is shown in figure-
1.
Pretreatment of water hyacinth: Fresh water hyacinth (leaves,
stem and root) on collection was chopped to small sizes of about
2 cm, allowed to dry under the sun for a period of 7 days, after
which they were dried in an oven at 600
C for 6 hours. The oven-
dried water hyacinth was then ground to fine particles using a
mixer grinder12
. Then sieve analysis of water hyacinth powder
was done using a sieve of mesh number 22.
Preparation of fermentation slurry: Based on material
balance 2880 g. of fermentation slurry with 8% total solids
content was prepared by mixing 112g. of pretreated water
hyacinth, 563.68g. of poultry litter and 2204.16 g. of water. 180
g. of resultant slurry was transferred to 300ml digesters of
different retention period. Each digester was given 5ml of 10%
by volume of acetic acid and 1.7g sodium bicarbonate.
Anaerobic co-digestion of digesters were carried out in
duplication till completion of their retention period in the
mesophilic temperature range.
Analytical methods: Solids analysis: Total solids (TS) and
volatile solids (VS) were determined for water hyacinth and
poultry litter according to standard methods13
.
pH analysis: pH was measured using a pH meter which
consisted of a potentiometer, a glass electrode, a reference
electrode and a temperature compensating device. Electrodes
were connected to the pH meter and were calibrated using
buffer solutions before pH analysis.
Integral analysis method: Integral method of analysis was
used to propose a kinetic model and determine the order and
rate constant of co-digestion of water hyacinth and poultry litter.
The reaction was assumed to be first order, rate for which is, (-
rA) = k CA
Where CA is the concentration of volatile solids.
Integrating with suitable limits gives rate equation-ln (CA/ CA0)=
kt. If plot of -ln(CA/ CA0) against‘t’ gives a straight line passing
through origin, then it can be inferred that the reaction follows a
first order kinetics with slope equal to rate constant k.
Otherwise assume a different rate expression and repeat the
procedure.
Results and Discussion
Solids and pH Analysis: Total solids (TS) were determined
after drying in oven overnight at 1050
C. Volatile solids (VS)
were determined by igniting the dried sample at 5500
C for 2
hours and determining the ash free dry weight. Total solids and
Volatile solids are calculated as given bellow.
Where, Wtotal is weight of dish and dried sample at 1030
C to
1050
C in grams. Wdish is weight of the dish in grams. Wsample is
weight of the dish and wet sample in grams. Wvolatile is weight of
the dish and sample after ignition at 5500
C in grams.
The total solids, volatile solids, and pH data of water hyacinth
and poultry litter are presented in table-1.
Table-1
Total solids,Volatile solids, and pH data
Material % TS % VS pH
Water Hyacinth 16.89 82.84 6.5
Poultry Litter 21.00 83.47 6.0
Biogas production: The biogas production ,total solids, volatile
solids, and pH were determined for each digester after the
expiry of their retention period; the values are tabulated in table-
2. The variation of specific biogas production, pH and solids
with time are presented in figures-2, figure-3 and figure-4
respectively.
Table-2
Cumulative biogas production, TS, VS, and pH data for all
the digesters
Digester Biogas
yield
(liters/gVS)
Total
Solids
(g)
Volatile
Solids
(g)
pH
WH-PL-0 - 14.4 11.88 7.0
WH-PL-7 0.0388 12.959 11.029 6.5
WH-PL-14 0.0494 12.626 10.797 6.5
WH-PL-21 0.1074 11.191 9.524 6.5
WH-PL-28 0.2172 8.362 7.117 6.5
WH-PL-35 0.3490 4.965 4.226 6.5
WH-PL-42 0.3664 4.517 3.845 7.0
WH-PL-49 0.3776 4.228 3.599 7.0
WH-PL-56 0.3810 4.187 3.564 7.0
From figure-2 it is clear that biogas produced was a function of
bacterial growth in the digesters and followed a sigmoid curve.
Distinctly three phases of microbial growth (lag, exponential
and death) can be visualized. The rate of specific gas production
was very less initially for 15 days which represents a greater lag
phase of hydrolysis. This gas production rate increased from15
to 40 days. Later the rate increased steadily to attain highest
biogas yield of 0.3810 l/g VS. pH is very important parameter
that controls biomethanation process. It can be inferred from
figure-3 that pH of the slurry was maintained in the neutral
range throughout the retention period which is essentially
because of the addition of buffering agent. It is seen from
figure-4, that volatile solids undergo biodegradation with the

digestion time to evolve biogas. Observation also reveals that
the rate volatile solid depletion is directly proportional to
microbial growth rate.
Kinetics of co-digestion: Integral method was employed to
determine the value of the rate constant for co-digestion of
water hyacinth and poultry litter. The plot of -ln CA/ CA0 versus
digestion time is shown in figure-5. This plot yields a straight
line passing through the origin which indicates that anaerobic
co-digestion of water hyacinth and poultry litter follows a first
order reaction. The slope of the straight line gives rate constant,
k. A ‘C’ program with method of least squares was written to
evaluate the rate constant, k which was found to be 0.026day-1
.
Figure-1
Experimental set up for Biomethanation
Figure-2
Cumulative biogas production

Figure-3
Variation of pH
Figure-4
Variation of TS and VS
Figure-5
Plot of -ln CA/ CA0 versus digestion time

Conclusion
From the study presented in this paper, the following
conclusions are made. Anaerobic co-digestion of water hyacinth
with poultry litter was explored and was found to produce good
amount of biogas (0.3810 l/g of VS). Variation of key
parameters (TS, VS & pH) with time was studied. Co-digestion
of water hyacinth and poultry litter follows first order reaction
with rate constant 0.026day-1
. However figure-5 shows a
negative gradient initially and a positive gradient towards the
end. These gradients are because of the three phases namely lag
phase, exponential phase and death phase. This gives further
scope for the researchers to understand the kinetics of individual
phases, which could explain the co-digestion of water hyacinth
and poultry litter in most appropriate way.
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